EP-A-0 180 020 and Microelectronic Engineering, Vol. 3, No. 1/4, 1985, pp. 397-410 discloses plasma etching processes simultaneously using ultra-high frequencies and radio-frequencies with a gaseous medium chosen from among halogens, rare gases, etc., in the case of EP-A-0 180 020, or a mixture of CF.sub.4 and oxygen in the case of the document.
These plasma etching processes simultaneously using ultra-high frequencies and radio-frequencies generally make it possible to obtain an improved result compared with that obtained with plasmas produced solely by radio-frequencies.
However, for producing deep structures in certain substrates, e.g. in silicon, in an industrial manner and with a good definition with respect to the geometry defined by the mask, it is necessary to further improve the results, particularly the etching speed or rate in order to have a high industrial production rate adapted to the etched depth and the control of the etched profile or anisotropy and the selectivity of the etching.
The characteristics of an etching made in a layer with the aid of a plasma are the speed or rate Vg, the anisotropy A and the selectivity S of the etching. The anisotropy of an eching A is determined by the ratio between the maximum width of the etching produced beneath the material constituting the mask of the etching and the depth of the etching. The smaller the anisotropy A, the more anisotropic the etching. The selectivity S of an etching corresponds to the ratio between the etching rate of the layer to be etched and the etching rate of the mask of the etching. The higher the selectivity S, the better the selectivity of the etching.